Recently, in high-frequency/high-capacity optical fiber communication systems, an optical modulator for which an optical waveguide-formed substrate is used and an optical transmission device into which an optical modulator as described above is combined are frequently used. Among them, optical modulators in which LiNbO3 (referred to as “LN”) having an electro-optic effect is used in a substrate are more broadly used in high-frequency/high-capacity optical fiber communication systems compared with modulators of a semiconductor-based material such as InP, Si, or GaAs. In an optical modulator for which this LN is used, an optical waveguide that guides light to the LN substrate in a confined manner is provided, and furthermore, electrodes that apply an electric field to the optical waveguide are formed. In addition, in the electrodes, an RF electrode portion that applies high-frequency signals, a DC electrode portion that applies low-frequency signals or DC voltages, and the like are formed.
Regarding the modulation form of optical modulators for high-frequency/high-capacity optical fiber communication systems, in response to the recent trend of an increase in the transmission capacity, multilevel modulation or transmission formats achieved by incorporating polarization multiplexing into multilevel modulation such as Quadrature Phase Shift Keying (QPSK) or Dual Polarization-Quadrature Phase Shift Keying (DP-QPSK) in which phase modulation is used has become mainstream, replacing On-Off keying or the like of the related art. Furthermore, it has been also proposed to form multiple elements using a plurality of DP-QPSK chips and further increase the transmission capacity (for example, refer to Patent Literature No. 1).
As illustrated in FIG. 1, in a DP-QPSK optical modulator, an optical waveguide (not illustrated) in which two nest-type optical waveguides including two Mach-Zehnder type optical waveguides are disposed is formed on a substrate (optical waveguide substrate) 1 of LN or the like. Furthermore, a plurality of signal electrodes 2 is provided on the substrate 1 in order to apply modulation signals to a modulation portion that the respective Mach-Zehnder type optical waveguides constitute. To each of the signal electrodes 2, modulation signals are input through a connector for electrical signal input 4. In addition, to a termination of the signal electrode 2, a termination resistor 5 is connected. In a case in which the termination resistor 5 is provided to each of the signal electrodes, there are cases in which a plurality of the termination resistors 5 is provided on the same termination substrate 3 as illustrated in FIG. 1, thereby reducing the size of the optical modulator. The substrate (optical waveguide substrate) 1 of LN or the like and the termination substrate 3 are disposed in a housing 6 and thus form a package.
In order to operate the optical modulator at a high frequency, a travelling wave-type electrode constitution in which electrical signals being input propagate through the signal electrodes is used. Signal frequencies being input to the signal electrodes are high-frequency signals in the microwave waveband, almost all of the input electrical energy is consumed in the termination resistors 5 and converted to heat in the termination resistors.
In the DP-QPSK optical modulator, four modulation portions are provided. In order to deal with the phase modulation form with the present constitution, the optical modulator is driven at a voltage magnitude being twice (an electric power being four times) as large as that of the on-off keying form of a single modulator structure of the related art. Therefore, the electric power being consumed in the modulator becomes 16 times or more as large as that consumed in a modulator having the single modulator structure of the related art. Furthermore, in order to meet a requirement of the size reduction of optical modulators, it is necessary to dispose the termination substrate 3 close to the optical waveguide substrate 1, which creates a significant problem attributed to heat being generated from the termination substrate.
Furthermore, in the case of forming multiple elements which intends to increase the transmission capacity by combining two or more DP-QPSK optical modulator constitutions into the same housing as illustrated in FIG. 2, the amount of heat generated becomes 32 times or more as large as that in the on-off keying form of the single modulator structure of the related art. Heat generated from the termination substrate deteriorates the temperature drift of the optical modulator. In addition, the generation of heat from the termination resistor causes the deterioration of the termination resistors over time or the occurrence of cracking, peeling, or the like and creates a serious problem of impairing the reliability of the optical modulator and optical transmission devices using the same. Meanwhile, in FIG. 2, optical waveguide substrates 1a and 1b are disposed side by side horizontally, but there are cases in which a plurality of optical waveguide substrates is disposed to be laminated together as in Patent Literature No. 1.
The influence of heat generated from the termination substrate has been an underlying problem of almost all of the optical modulators having a travelling wave-type electrode constitution; however, in the related art, this problem has been rarely studied or dealt with. Instead, the influence of heat generated has been misunderstood as a temperature change of an environment in which the optical modulator is placed or the unstability of the optical modulator and has been handled as a problem of the deterioration of the intrinsic characteristics of the optical modulator such as temperature drift.
However, this influence has become particularly significant in (a) an optical modulator in which the amplitude of input electrical signals is large, (b) an optical modulator having a plurality of termination resistors, and (c) an optical modulator in which termination resistors are on the same substrate such as optical modulators having the DP-QPSK constitution. Furthermore, this influence has become more serious as the optical modulator has been reduced in size (d) and has been provided a plurality of elements (multiple elements) (e).
As a countermeasure for alleviating the above-described problem of heat generated from the termination resistors, it has been proposed to increase the area of the termination resistors or provide a heat-conductive hole in the termination substrate as described in Patent Literature No. 2. However, in these constitutions and methods, the termination substrate becomes large, and the manufacturing costs also increase, and thus the applicable usages are limited. Therefore, there has been a desire for a solution which can be applied to a variety of transmission formats and meets the requirement of size reduction or cost reduction. In addition, there is a demand for a highly-reliable optical transmission device in which the temperature drift is suppressed by mounting an optical modulator to which a countermeasure to heat generated is provided.